Attachment plug



J. H. EISNER ATTACHMENT PLUG Oct. 11, 1949.

2 Sheets-Sheet 1 Filed Oct. 25, 1947' INVENTOR JOHN H. EISNER Y E N R O n A Oct. -11, 1949. J. H. EISNER 2,484,558

ATTACHMENT PLUG Filed Oct. 25, 1947 2 Sheets-Sheet 2 INVENTQR JOHN H. EISNER ATTORNEY Patented Oct. 11 1949 UNITED STATES PATENT OFFICE ATTACHMENT PLUG John H. Eisner, Auburn, N. Y.

Application October 25, 1947, Serial No. 782,122

9 Claims. I

This invention relates to electric attachment plugs and more particularly to three-wire safety plugs for use with conventional two-wire plug receptacles.

In the use of household electric appliances such as washing machines, vacuum cleaners and the like; and in the use of small electric hand tools in shops and garages; diificulty has been experienced due to the exposed metal framework of such apparatus becoming electrically charged by contact with some conductor used in connection therewith and thereby subjecting the user of such equipment to electric shock as well as constituting, in itself, a fire hazard.

Household electric appliances and small electric hand tools derive their power thru two insulated wires usually bound together in a cord. Each wire is attached to one contact element of a two-way plug adapted to connect with complementary, electrically charged contact elements or" a plug receptacle contained in a metal outlet box built into the wall or floor of the building. Such plugs today are almost universally of a standard type consisting of two parallel blades substantially tangential to a circle of standard diameter centering in the plug. Receptacles are likewise standard in a complementary sense so that a plug of standard design will fit in any receptacle cf standard design. Such receptacles may be single or duplex. Duplex receptacles have two pairs of parallel slots to receive the blades of two plugs at the same time so that more than one appliance or tool may be plugged simultaneously into the same receptacle. In almost a11 duplex receptacles the two pairs of slots are disposed on either side of the center of the receptacle at a standard distance from such center, all slots being parallel to a line between the centers of the plugs when the plugs are plugged into the receptacle.

It is standard practice, today, to support the receptacle in the metal outlet box by means of a metal strip or bracket forming part of the receptacle, and which is attached by means of screws to the outlet box. Likewise, a face plate is provided, substantially flush with the wall into which the outlet box is built, and affixed to the receptacle by a single, centrally located screw which is threaded into a metal portion of the receptacle supporting bracket. The face plate retaining screw is thus directly connected electrically to the metal outlet box. In many buildings, the wiring is of metal armored cable or metal conduit type and such armor or conduit is grounded by connecting to the water system or otherwise. Since the metal armor or conduit is clamped to the outlet box, the latter is, accordingly, also grounded. Where cable covered with insulating material is employed, the outlet box may be grounded by a special third wire.

The purpose of this invention is to provide a connecting means in the form of an attachment plug which will effectively ground the electric apparatus, appliance or tool to the outlet box through the receptacle bracket, before connecting such electric apparatus, appliance or tool to the source of electric current.

More specific objects of the present invention are to provide an attachment plug which'will prevent any possibility of connecting such electric apparatus, appliance or tool, to the source of electric current before said ground connection is made; and which will also prevent any possibility of breaking said ground connection before breaking the electric power connection between the source of electric current and such electric apparatus, appliance or tool.

Another object is to provide an attachment plug which may be used without the grounding feature in connection with any conventional two-wire plug receptacle; and with the grounding feature in connection with any conventional two-wire duplex receptacle by a simple; easily-made adaptation of said receptacle and without preventing the ordinar double use of said duplex receptacle.

Further objects are to provide an attachment plug which is simply and cheaply made, which is not unusual in appearance and which will not detract from the usual appearance of the appliance or tool with which the present invention is used and which yet has the protective features hereinbefore referred to.

Efforts in the past to provide a means of grounding household electric appliances or portable electric tools have resulted in three-way attachment plugs which can only be used with three-way receptacles, or in three-way attachment plugs in which the third, or ground, wire is attached by a separate action than that used in connecting the appliance or tool to the source of power. Disadvantages of these two methods are apparent in that special three-way receptacles are expensive to install and limit the use of the appliance or tool to use with converted receptacles only; while three-way attachment plugs which have a special, separate connection for the ground wire, are not truly safety plugs, since connection to the source of power may be made before the ground connection is made, or the ground connection may be broken before the power connection.

Embodiments of the present invention are shown in the accompanying drawings, in which:

Figure l is a side elevation, partly in section on the line li in Figure 2, showing two plugs and a ground pin in cooperation with a conventlonal duplex receptacle which is indicated in not and dash lines;

Figure 2 is a plan view thereof;

Figure 3 is a bottom plan view of one plug;

Figure 4 is a side elevation of a modified form or the invention, showing two plugs and a different rorm of ground pin in cooperation with a conventional duplex receptacle which is indicated in dot and dash lines;

Figure 5 is an enlarged, fragmentary, sectional view of the ground pin and adjacent parts shown in Figure l on the line 5-5 in Figure 6;

Figure 6 is a sectional view of the ground pin alone, taken on the line 6-6 in Figure 5; and

Figure '7 is a plan view of the screw shown in Figure 5.

Referring now more in detail to Figures 1, 2, and 3 of the drawing, an electric attachment plug embodying the improvements made in accordance with the present invention is shown as comprising a body member ID of molded insulating material, having a recess H at one end thereof and a centrally disposed opening 12 therethrough to receive the conductor wires, usually bound three together in a cord, for electrical appliances or equipment with which the said attachment plug is to be used. Mounted in the recess H are the spaced contact elements l3 and i4 formed with contact blades I5 and I6 respectively, and which are provided with terminals or binding posts l? and 18 respectively, to which the ends of two of the conductor wires may be connected in a conventional manner. These contact elements E3 and i i are fixed in relation to the body member it with the blades [5 and i6 thereof in substantial parallel relation and substantially tangential to a circle centering in the plug, and may be used in a conventional manner with a twowire receptacle such as 19 indicated in Figure 1.

Recess II has a portion 20 more deeply recessed in which is mounted a third contact element 2! provided with a third terminal or binding post 22 to which may be connected in a conventional manner the end of a third conductor wire or ground wire which is adapted to be connected to the metal parts of the appliance or equipment with which the said attachment plug is to be used. Contact element 2| is spaced in body member away from contact elements 13.

and I l and metal parts in contact therewith, and terminates in the hollow, cylindrical contact sleeves 23 and 24 which are fixed within the radially extending offset arms 25 and 26 respectively, of body member ID.

Arms 25 and 26 are radially disposed about the center vertical axis of the plug so that their axes li along a line perpendicular both to the center vertical axis of the plug and to a line drawn between centers of blades and I6, and each has a centrally disposed cylindrical opening, 25a and 2612, respectively, parallel to the central axis of the plug, and coincident with the openings of the cylindrical contact sleeves 23 and 24, respectively, of such size as to snugly admit the ground pin 21.

The centers of said cylindrical openings 25a and 26a are each equally at such a distance radially from the center of the plug that when two plugs are inserted in a standard conventional duplex receptacle, one of said centers in each plug 4 lies vertically over a point midway between the centers of said plugs, which point corresponds to the location of the conventional face plate screw.

The metal ground pin 21 is longitudinally slotted as at 42 to provide easy admission to openings 23a or 26a and to insure firm resilient contact with the inner metal surface of the cylindrical contact sleeve 23 or 24. The lower portion of ground pin 27 is threaded as at 23 to permit its substitution for the face plate screw, which secures the face plate, such as indicated at 3!, to the body of the receptacle I3. Shoulder 29 on the pin 21, and lock washer 30, hold the face plate 3! to the receptacle l9. It will readily be seen that the substitution of ground pin 27 for the face plate screw will place it in such position that opening 25a or 27a will be aligned with and will fit over the ground pin 2'! when the plug is inserted in the receptacle. Since the face plate screw of the conventional receptacle is grounded as above described, it will be apparent that when my improved electric attachment plug is used, the metal framework of the appliance or tool will be grounded through the third wire of the electric cord, binding post 22, contact element 2|, contact sleeve 23 or 24, and ground pin 21 to the ground of the wiring system.

Arm 25 is vertically ofiset above arm 26 a slightly greater distance than the thickness of arm 25, so that two plugs may be used, each providing safety ground connection, in any duplex receptacle which has been adapted by the substitution of ground pin 2'! for the ordinary face plate screw. The first plug is plugged into the receptacle so that arm 25 is over the ground pin 21; then the second plug may be plugged in with its arm 26 over the ground pin.

The ground pin 2! is preferably of such length that contact must be made between contact sleeve 23 or 24 and pin 21 before blades 15 and I6 can make contact with their respective spring contact elements in the receptacle l9.

It will be understood that variations of construction may be resorted to within the scope of the invention. An example of a modified construction is illustrated in Figures 4, 5, 6, and 7, which show a ground pin in the form of the contact cartridge 32 substituted for the screw ground pin 27.

Ground contact cartridge pin 32 is shown in Figure 5 as comprising a body portion 33, and lower or contact section 36 which telescopes longitudinally into and within body portion 33. Both parts are substantially cylindrical and surround coil spring 38, which urges the contact section 36 longitudinally outward from within body portion 33; the latter having a closed upper end while the contact section 36 has a closed lower end. The body portion 33 has a doubly flanged lower end as at 34 and 35. Exterior flange 34 fixes the position of contact cartridge body 33 when it is pressed into cylindrical opening 26a in arm 26 where it is retained by a tight fit. Interior flange 35 centers and retains the contact section 36 within body portion 33, in cooperation with the exterior flange 31 at the upper extremity of contact section 36. Flanges 34 and 35 may be formed in any suitable manner, such as by bending adjacent sections of the wall of cartridge body 33 alternately in and out as shown in Figures 5 and 6.

Cylindrical opening 25a or arm 25 of another plug may be slid freely but with firm metal to metal contact over body portion 33, without dislodging contact cartridge 32 from its position in cylindrical opening 26a of arm 26, by reason of longitudinal slots such as 39 in the side wall of the upper end of cartridge body 33 which extend downward only so far as the upper surface of arm 26.

It will be apparent that contact cartridge 32 is carried in arm 26, in metal to metal contact with contact sleeve 24; and will maintain contact with the grounded wiring system, when the plug is plugged into the receptacle, through contact of its lower or contact section 36, as it is extended by spring 38, with the center screw 50 holding the face plate 3i to the receptacle body.

As another feature of the present invention, the coil spring 38 is of a capacity sufficient to hold lower section 36 in firm contact with the face plate screw 4i! but not of sufficient capacity to disengage blades l5 and W from their contact with their respective spring contact elements in the receptacle.

Screw 40 (best shown in Figures 5 and 7) may be substituted for the conventional face plate retaining screw to provide better contact through the conical hole t! in its face surface which matches the conical lower end of contact cartridge lower section 36.

In cooperation with receptacles having a metal face plate which is grounded in any manner to the wiring system, the present invention may be used with contact cartridge 32 without regard to the location of the face plate retaining screw, as shown, for example, at the left side of Figure 4. In such case, the arm 25 may be omitted.

To those skilled in the art, it will also be apparent that ground pin 2'1 could be made with its upper portion blade-shaped similar to blades l5 and I6; and that contact sleeves 23 and 24 could then be made similar to the complementary spring contact elements commonly used in conventional plug receptacles. Such a modification has not been illustrated since the construction shown is deemed preferable. It will also be understood that instead of using screw type binding posts l1, l8 and 22, the three conductor wires may be soldered directly to the associated contact elements.

While I have herein described and in the drawings shown illustrative embodiments of the invention, it is to be understood that the invention is not limited t'hereto, but may comprehend other constructions, arrangements of parts, details and features without departing from the spirit of the invention.

I claim:

1. An electric attachment plug adapted for use with a grounded electric plug receptacle, and comprising a body member of insulating material having a rigid arm projecting radially therefrom, a pair of spaced contact elements mounted in said body member and having blades adapted for engagement with terminal members of the plug receptacle to complete an electric circuit, a third spaced contact element mounted in said body member and having a ground pin-engaging portion located in said radial arm, and a metal ground pin constructed and arranged for automatic simultaneous connection upon insertion of said plug in the plug receptacle with a grounded portion of the plug receptacle and said portion of said third contact element.

2. An electric attachment plug adapted for use with a grounded electric plug receptacle and comprising a body member of insulating material having a rigid arm projecting radially therefrom, a pair of spaced contact elements mounted in said body member and having blades adapted for engagement with terminal members of the plug receptacle to complete an electric circuit, a third spaced contact element having a ground pin-engaging portion located in said radial arm, said portion having an opening therethrough, and a metal ground pin having one end adapted to fit in said opening through said portion of the third element and cooperating therewith and having its other end in contact with a grounded metal portion of said receptacle when the plug is inserted therein, whereby a ground connection is made automatically and simultaneously upon insertion of said plug in said plug receptacle.

3. For use with a grounded electric plug receptacle, an electric attachment plug comprising a body member of insulating material having a rigid arm projecting radially therefrom, spaced contact elements mounted in said body member and having blades adapted for engagement with terminal members of the receptacle to complete an electric circuit, a third spaced contact element having a hollow cylindrical contact sleeve mounted in said radially projecting arm, and a metal ground pin having one end substantially cylindrical so as to be contained within and contacted by said contact sleeve and having its other end in contact with a grounded metal portion of said receptacle when the plug is inserted there in, whereby a ground connection is made automatically and simultaneously upon insertion of said plug in said plug receptacle.

4. For use with an electric plug receptacle having a face plate retained by a screw threaded into a grounded metal portion of said receptacle; an electric attachment plug comprising a body member of insulating material having a rigid arm projecting radially therefrom; two spaced contact elements mounted in said body member and having blades adapted for engagement with terminal members of the receptacle to complete an electric circuit; a third spaced contact element having a hollow cylindrical contact sleeve mounted in said radially projecting arm, at such a radial distance from the center of the plug as to be centered over the face plate retaining screw hole of the receptacle, when the plug is inserted therein; and a metal ground pin having an upper slotted portion which resiliently contacts and is contained within the hole through said sleeve, when the plug is plugged in, and having a lower end threaded into the grounded face plate retaining screw hole of the receptacle, whereby a ground connection is made automatically and simultaneously upon insertion of said plug in said plug receptacle.

5. For use with an electric plug receptacle having a grounded metal portion at the face thereof; an electric attachment plug comprising a body member of insulating material having a rigid arm projecting radially therefrom; two spaced contact elements mounted in said body member and having blades adapted for engagement with terminal members of the receptacle to complete an electric circuit; a third spaced contact element having a hollow cylindrical contact sleeve mounted in said radially projecting arm at such a radial distance from the center of the plug as to be located over a grounded metal portion of the face of said receptacle when the plug is inserted therein; and a cylindrical, metal, ground pin, comprising a body portion which contacts and is firmly retained within said sleeve, a contact-portion telescoped Within said body portion, and a spring contained Within said body por- 7" tion and which urges said contact portion to project toward and to contact a grounded metal portion of said receptacle when the plug is inserted therein, whereby a ground connection is made automatically and simultaneously upon insertion of said plug in said plug receptacle.

6. An electric attachment plug, for use, singly or in pairs, with a grounded duplex electric plug receptacle; said plug comprising a body member of insulating material, two spaced contact elements mounted in said body member and having blades adapted for engagement with terminal members f the receptacle to complete an electric circuit, a third spaced contact element and a metal ground pin cooperating therewith; said body member having two diametrically opposite rigid arms projecting radially from the center of the plug in such direction that one of said arms overlies the center of the receptacle when the plug is inserted therein, said arms being ofiset relatively to each other axially of the plug a distance such that the bottom of one arm is spaced from the top of the other; said third contact element having ground pin-engaging portions located in said projecting arms, each said portion being at a radial distance from the center of the plug half the center distance between plugs when two plugs are inserted in the receptacle; and said cooperating metal ground pin being constructed and arranged for automatic simultaneous connection, upon insertion of said plug in the plug receptacle, with a grounded portion of said receptacle and one of said portions of the third contact element.

7. An electric attachment plug, for use, singly or in pairs, with a grounded duplex electric plug receptacle; said plug comprising a body member of insulating material, two spaced contact elements mounted in said body member and having blades adapted for engagement with terminal members of the receptacle to complete an electric circuit, a third spaced contact element and a metal ground pin cooperating therewith; said body member having two diametrically opposite rigid arms projecting radially from the center of the plug in such direction that one of said arms overlies the center of the receptacle when the plug is inserted therein, said arms being ofiset relatively to each other axially of the plug a distance such that the bottom of one arm is spaced from the top of the other; said third contact element having ground pin-engaging portions located in said radial arms at a radial distance from the center of the plug half the center distance between plugs when two plugs are inserted in the receptacle, said portions each having an opening therethrough; and said ground pin having one end adapted to fit in said openings through said portions of the third element and having its other end in contact with a grounded metal portion of said receptacle when the plug is inserted therein, whereby a ground connection is made automatically and simultaneously upon insertion of said plug in said plug receptacle.

8. An electric attachment plug, for use, singly or in pairs, with a grounded duplex electric plug receptacle having a face plate held in positon by a centrally located screw which threads into a grounded member of said receptacle; said plug comprising a body member of insulating material, two spaced contact elements mounted in said body member and having blades adapted for engagement with terminal members of the receptacle to complete an electrical circuit, a third spaced contact element and a metal ground pin cooperating therewith; said body member having two diametrically opposite rigid arms projecting radially from the center of the plug in such direction that one of said arms overlies the center of the receptacle when the plug is inserted therein, said arms being ofiset relatively to each other axially of the plug a distance such that the bottom of one arm is spaced from the top of the other; said third contact element having similar hollow, cylindrical contact sleeves mounted in said projecting arms of the body member, each said sleeve being at a radial distance from the center of the plug half the center distance between plugs when two plugs are inserted in the receptacle; and said ground pin having a lower portion threaded and adapted to replace the central face plate retaining screw of the receptacle, and having a slotted, upper portion substantially cylindrical so as to be adapted to be contained within, and in resilient contact with, the said sleeve of one arm of said plug when the plug is inserted in the receptacle, whereby the said ground pin holds the face plate in position and grounds said sleeve to the grounded member of the receptacle.

9. An electric attachment plug, for use, singly or in pairs, with a grounded duplex electric plug receptacle having a face plate held in position by a centrally located screw which threads into a grounded member of said receptacle; said plug comprising a body member of insulating material, two spaced contact elements mounted in said body member and having blades adapted for engagement with terminal members of the receptacle to complete an electric circuit, a third spaced contact element and a metal ground pin cooperating therewith; said body member having two diametrically opposite rigid anns projecting radially from the center of the plug in such direction that one of said arms overlies the center of the receptacle when the plug is inserted therein, said arms being offset relatively to each other axially of the plug a distance such that the bottom of one arm is spaced from the top of the other; said third contact element having similar hollow, cylindrical contact sleeves mounted in said projecting arms of the body member, each said sleeve being at a radial distance from the center of the plug half the center distance between plugs when two plugs are inserted in the receptacle; and said ground pin comprising a body portion which contacts and is firmly retained within the said sleeve of the lower projecting arm, a contact portion telescoped within said body portion, and a coil spring contained within said body portion and which urges said contact portion to project toward and to contact a grounded metal portion of said receptacle when the plug is inserted therein.

JOHN H. EISNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,067,796 Smith Jan. 12, 1937 2,323,736 Tousley July 6, 1943 FOREIGN PATENTS Number Country Date 162,276 Switzerland Aug. 16, 1933 639,630 France Mar. 13, 1928 

